It was the 100th story of Doctor Who since its revival in 2005. This episode introduces a new warrior alien race, known as the Mire; much like Sontarans, they revel in battle. A new immortal is introduced, Ashildr. It also marks one of the few mistakes that the Doctor has made when he's emotional.

This story also once more shows how easily aliens were able to manipulate humans before they learned of the possibility of life beyond the stars; to them any advanced technology is magic. The Mire leader posed as Odin, tricking the people into not fighting when his soldiers harvested fighters for sustenance.

The story was notable for revealing the origins of the Twelfth Doctor's face: he had unknowingly taken on the likeness of Lobus Caecilius, a man whose life the Tenth Doctor had saved. This marked the first time Doctor Who had given an explanation for the Doctor being played by an actor who had previously played a different role in the series. Colin Baker, who first portrayed the Time Lord Maxil, later went on to portray the Sixth Doctor; however, in this instance, it was never explained in-universe why the Sixth Doctor had taken on Maxil's appearance, or if the likeness was deliberate.

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After an adventure and a half in space, the Doctor and Clara are kidnapped by 9th-century Vikings. However, to make matters worse, hostile aliens have also arrived in the vikings' village; they are provoked into declaring war on the village by a stubborn girl.

By the end of the adventure, the Doctor will learn where it was that he saw his own face before, and the reason why he chose it.

Clara is floating in space, desperately calling the TARDIS for a pickup. The Twelfth Doctor answers, but says he's too busy at the moment to help as the TARDIS is under attack by the hostile aliens attacking the Velosians. Clara then notices that something is in her spacesuit; the Doctor suggests it's a Love Sprite, a parasite that eats brains from out of the mouth, hence the name. The Doctor points out it's because she spent too long in the spider mines. He then asks her to list the four most interesting stars she can see: two whisky-coloured stars below a blue one and a green wing-shaped nebula.

Within moments of the Love Sprite getting too close to Clara's head, the Doctor materialises the TARDIS around her and takes off her helmet. Clara breathes heavily in relief. The Doctor chases the creature, squashing it under-foot. Clara wonders how things worked out on his end. The Doctor retorts he just saved an entire planet by transporting their attackers half-way across the galaxy and draining their weapons systems; not to mention saving a school teacher from suffocating/having her brain eaten.

The Doctor decides to land the TARDIS to clean his boot in the grass; he steps outside into a forest, bracing himself against a tree as he wipes the Love Sprite off his boot. Clara wonders what's to stop the aliens from attacking again, to which the Doctor says "nothing. At least this time, they'll be ready for them. I'm not actually the police, Clara; that's just what it says on the box." He explains that as time travellers, it's okay to make ripples, not tidal waves; Clara laughs, calling him a tidal wave. Realising that was a jinx, he retorts not to say that.

At that moment, they find themselves held at sword-point. The Doctor moans about not being in the mood for vikings, who surround them. The leader tells the Doctor that he and Clara are coming with them. Filled with his usual confidence/arrogance, the Doctor puts on his sonic sunglasses, gloating that what he wears on his face is technology more advanced than anything the human race will manage in the next nine million years. Unimpressed, the leader simply takes the sonic and breaks them in half. Seeing that he should have had a backup plan, the Doctor flatly tells Clara that they're going with the Vikings.

Two days later, the Doctor and Clara are dragged by the Vikings back to their village, enchained. For the last two days, which was spent on a rowboat, the Doctor has been lying to Clara about having a plan, to keep her calm. However, Clara has travelled with him long enough to know when he's lying. The leader gifts a girl named Ashildr with the right half of the Doctor's sonic sunglasses; she seems to think it's a weird eye patch. Clara wonders if the Doctor has a plan now, to which he says is the usual - "find the boss man, replace him"; he recites Clarke's Law to Clara, who wonders "is it going to be the yo-yo again?" To her surprise, she finds the Doctor has already freed himself of his cuffs using what he calls "magic".

The lead Viking addresses his father, whom the Doctor takes to be the village leader. Tossing his yo-yo at the Viking, the Doctor is met with blades pointed at him. The Doctor loudly declares that he's cross with the Vikings and that he's very, very disappointed in them; he tells them that he's taken human form to walk among them. When a villager asks for his identity, the Doctor tosses his yo-yo upwards, yelling "Surely, you would recognise the sign of Odin." The Vikings declare that the Doctor is not Odin and the yo-yo is not Odin's sign, to which he asks if they have actually met Odin and know what he looks like. As if responding to the Doctor's words, what seems to be Odin's face appears in the sky. Odin declares that the day of reward has come for the villagers; the Doctor yells "Do not believe this impostor" as he fails at a yo-yo trick, embarrassing himself.

A better Odin shows up.

Odin declares the best warriors will dine with him in Valhalla. Clara wonders if it's really Odin, to which the Doctor says "he doesn't even have a yo-yo" - no, it's not Odin, just a better fake. Armoured aliens teleport down, and begin scanning the villagers. They teleport the strongest warriors away. The Doctor explains they're not invading, but harvesting; children and the weak will be ignored. Outraged, Clara wants to stop them, but the Doctor tells her they need to avoid being chosen as this will let them escape. Ignoring him, Clara rushes over to Ashildr, whom she instructs on how to use the sonic sunglasses to unlock her cuffs. Unfortunately, the advanced tech is noticed, they are both teleported to a ship hidden behind Odin's projection.

On board the ship, Clara notices that her cuffs are gone and that the warriors are impatient to meet Odin. The leader tries opening a door in front of them, succeeding only to be zapped into dust in the next room. A wall begins pushing everyone into the chamber, despite their best efforts to jam it with their blades. Clara and Ashildr manage to open the door on the other end of the chamber, barely escaping the next zap that reduces the other warriors to dust as well.

On Earth, Ashildr's father, Einarr, notices that all the chosen went willingly to Valhalla, while another says he wouldn't due to his fear of heights. Annoyed at their stupidity and superstitions, the Doctor tells them that homo sapiens are an intelligent species, so they should stop lying to themselves; they just got raided. Einarr tells the "false Odin" to choose his words carefully, to which the Doctor admits that he lied, but "the big fella in the sky" lied too "because what's the one thing Gods never do? Gods never actually show up!" When the Doctor says he lost someone important, Einarr retorts that he did as well.

Odin questions Clara.

Back on the ship, Clara and Ashildr wake to find themselves in what appears to be a factory. Ashildr wonders why they are still alive. "Odin" arrives with two aliens, saying it's because of the sonic sunglasses; he examines them in his hand. He asks Clara to explain. Taking the Doctor's role, Clara says she's sorry for scaring the aliens, to which "Odin" denies. Clara knows he's examined the sunglasses and determined that they have from a civilization far more advanced the village, not to mention she's wearing a spacesuit. An alien brings "Odin" a vial of green liquid, to which Clara quips "time for your medicine?" "Odin" calls it nectar, downing in a gulp; it was made from the warriors' adrenaline and testosterone; Clara is puzzled as to why aliens travelled to Earth to "mash-up" some Vikings for "warrior juice". She asks "Odin" why he played God to Vikings, to which he says there's always some kind of "farmer" that "cattle" will respond to.

Clara calls "Odin" a thief caught in the act, asking him to seek warriors elsewhere for his sustenance as "the universe is full of testosterone; believe me, it's unbearable." She then asks "Odin" if he really wants to start a war, seeming to make him consider leaving peacefully. However, Ashildr butts in, declaring revenge. "Odin" laughs, saying he was almost fooled into leaving as "words are the tools of cowards." He tells them that ten of his warriors will attack tomorrow. Surprised "Odin" is being so idiotic, Clara asks for his reason. "Odin" says it's for "the joy of war"; his species live for the battle. He then removes his hologram face, to show his joy. Ashildr is left shocked that she has declared war on a monster.

Back in the village, the Doctor reads his 2000 Year Diary, learning the aliens are the Mire, who leave others alone if they get what they want; they are one of the deadliest warrior races in the galaxy. Clara returns at that moment, explaining Ashildr's blunder. The Doctor tries explaining that the best option for the villagers is to run since the Mire will give up on trying to kill them if they hide out for about a week. However, he fails due to their pride; upon hearing a baby's cries, which the Doctor can understand as words, he decides to stay and teach the villagers basic combat. He nicknames them as he hates to waste time learning names. Unfortunately, they are too blundering or questioning to be properly trained, resulting in accidents. Clara tells the Doctor to stop playing soldier and start looking for what he's missing - how he's going to win - because he always finds it at the last minute.

The Doctor comes across Ashildr, who is fighting a puppet of Odin. Ashildr explains she makes puppets and comes up with stories whenever the raiding parties head out, believing the right story will ensure their safety. She further elaborates about never really fitting in with boys or girls, due to the girls all thinking she's a boy and the boys all thinking she's just a girl. Her father "Chuckles" consoles her, to which the Doctor allows them privacy. However, he then notices "Lofty" is stealing the baby. Ashildr explains it's Lofty's child; he takes her to the boathouse to see the fish when she won't settle; the Doctor immediately realises why, calling for Clara as he rushes to the boathouse. The Doctor examines the buckets as Clara enters; she wonders if he got his hand stuck in something again because he's yelling. The Doctor explains he's found what he's missing and that no-one told them they had "fire in the water" - electric eels; telling Chuckles to inform everyone that bedtime is cancelled, the Doctor goes on to announce "There's going to be a war tomorrow. This just in: we're going to win the Hell out of it!"

They spend the entire night prepping for battle; the Doctor tells the group that they should look happier than their enemies as it assures winning and to always walk briskly as it makes them moving targets. He also adds to act like they know their enemies' plans, as sometimes, very rarely, they will be goaded into revealing them. Among the various items mentioned for the plan are anvils and the silver wiring from Clara's spacesuit. After the prep-work is done, the Doctor explains the plan - if they can get him a helmet from a Mire soldier, this entire thing is over; they can mop up the rest with Ashildr's monstrosity. The moment it's unveiled, Clara points out it's "rubbish", to which the Doctor smirks "I know".

The next day "Odin" arrives with some of his men, they enter a barn, where the Doctor, Clara and the villagers are all dancing and having fun. When Odin points out that it's time to fight, the Doctor says they decided to have a party and there's not a single weapon in the barn. "Odin", desiring a fight, threatens to kill everyone as it wouldn't be the last time the Mire caused a senseless slaughter of innocent and unarmed people. During this time, Lofty has been tossing rings on the Mire suits, failing at six; the Mire are briefly startled.

On the Doctor's cue, Chuckles startles a bucket of eels; their electric charge is channelled into the barn by the wiring taken from Clara's spacesuit, rendering six of the Mire suits immobile. The villagers run out as the Doctor cues Chuckles to startle the next bucket, which turns an anvil hanging above the Mire into a magnet that takes their helmets and blasters.
Yelling for the magnet to be turned off, the Doctor takes a helmet, while Clara takes a blaster to hold the Mire at bay. The Doctor quickly gives the helmet to Ashildr, who uses the visual link to trick them into thinking a puppet of a sea monster is a real one attacking them. All but "Odin" teleport back to the ship. Ending the ruse, the Doctor shows "Odin" the recording of this embarrassment on Clara's phone, which has been made funnier by adding "The Benny Hill Theme"; he threatens to upload it to the Galactic Hub (and thus ruin the Mire's reputation) if they don't leave peacefully. "Odin" threatens the Doctor's actions will not go unpunished but is teleported back to his ship by the Doctor, who hacked the system.

The Mire flee.

Without any further conflict, the Mire leave the Earth; the Doctor will likely keep a copy of their humiliation to keep them from any more senseless fights. The villagers rejoice in their victory. However, the Doctor discovers that using the Mire helmet has drained Ashildr's heart, leaving her dead; he had not foreseen it would take such a toll.

The Doctor is left brooding about the loss, with Clara trying to comfort him, saying he couldn't save her. He snaps back, saying he can do anything, but the Laws of Time prevent him from doing so. Looking at his reflection, the Doctor suddenly remembers back to when he first saw it, wondering once more "who frowned me this face?" Suddenly, the memory comes back to him; Donna Noble encouraged him to always try to at least save someone even in a fixed point, and he saved Lobus Caecilius' family from Pompeii's destruction. He tells Clara that the reason his subconscious chose this face when he regenerated was to remind him to always save someone, no matter how impossible or wrong it seemed. The Doctor then yells to the sky:

I'm the Doctor, and I save people. And if anyone happens to be listening, and you've got any kind of a problem with that, to hell with you!The Doctor

Rushing back to the barn, the Doctor fiddles with the helmet and pulls out a chip. He puts it on Ashildr's forehead and activates it, bringing her back to life; it's the medical kit for the Mire soldiers. The Doctor gives her father a second chip, telling him it's for whomever she wants; he explains to Clara that the kit would likely keep repairing her, preventing Ashildr from dying. The second chip is to prevent her from being alone for eternity.

Returning to the TARDIS, the Doctor is left wondering if he did the right thing; when he's emotional, he makes mistakes. Clara assures him everything will be fine because what he did was born of good intentions. Brooding again, the Doctor says, "Time will tell, it always does." However, he then realises that his actions have turned Ashildr into a hybrid.

Ashildr is then seen in a montage of time passing her by, but with age never affecting her. As the screen pulls into her face, she is shown to have stopped smiling, her smile being replaced with a grim stare...

Not every person who worked on this adventure was credited. The absence of a credit for a position doesn't necessarily mean the job wasn't required. The information above is based solely on observations of the actual end credits of the episodes as broadcast, and does not relay information from IMDB or other sources.

The Radio Times programme listing was accompanied by a small colour head-and-shoulders shot of Ashildr dressed in armour during the Battle of Agincourt, as seen in flashback during the following episode, The Woman Who Lived, with the accompanying caption "Doctor Who / 8.20 p.m. / Viking girl Ashildr (Maisie Williams) becomes the focus of the Doctor's attention".

The clock gear tunnel at the start of the opening titles is slightly darker in this episode.

The Doctor and his companions previously tried to resolve conflicts peacefully, only to have their efforts made pointless. (TV: The Poison Sky, Cold War, etc)

The Doctor attempts to "reverse the polarity of the neutron flow," a statement he used often during his third incarnation, and would occasionally be used by later incarnations. (TV: The Sea Devils et.al.) Here, he claims the phrase is actually meaningless.

The Doctor had previously wondered where he had gotten his face, and had theorised he was trying to tell himself something. (TV: Deep Breath) He finally realises his face matches that of Lobus Caecilius and that the message he was telling himself was to save a person, even if it meant breaking the rules of time. (TV: The Fires of Pompeii)

Upon thinking about how Ashildr's situation will play out now that she is functionally immortal, the Doctor says, "Time will tell. It always does." His seventh incarnation said this to Ace when she hesitantly asked if they "did good" after the Shoreditch Incident, aware that their victory over the Daleks came at the expense of many lives. (TV: Remembrance of the Daleks)

The Doctor states that Ashildr is immortal "barring accidents," echoing a statement he previously made in reference to the Time Lords. (TV: The War Games)

The Doctor worries that saving Ashildr's life will have a negative effect on time and that he did it because he had gotten emotional. In his tenth incarnation, he interfered with a fixed point in time (TV: The Waters of Mars), as River Song also did, and ended up causing more harm than good in altering the flow of history. (TV: The Wedding of River Song) Both of these instances were influenced by putting their own personal feelings above the consequences of their actions.

The Doctor notes how small changes in the course of time can go from ripples to tidal waves. In his seventh incarnation, he once made a similar comparison to the consequences of choices individuals make, saying, "Every great decision creates ripples, like a huge boulder, dropped in a lake. The ripples merge, rebound off the banks in unforeseeable ways. The heavier the decision, the larger the waves, the more uncertain the consequences." (TV: Remembrance of the Daleks) The Doctor had also recently compared changing time to ripples in a pond in TV: Before the Flood.

Contrary to common belief, season 10 kicked off in the last week of December 1972 — not in 1973, as would be expected. Season 10 actually began nine years after season 1 started. In fact, The Three Doctors began nine years to the week after The Daleks first aired.

For the purposes of this list, "Series 4" is considered to be the production series 4, which ran all the way from Time Crash to The End of Time.

The years seen in this section may seem decidedly "off". Remember, however, that this list only gives the first year in which an episode from a series was broadcast. David Tennant, unusual amongst other Doctors, began and ended on special episodes, not regular ones. Thus, his series actually begin in 2005, 2006 and 2007 — not 2006, 2007 and 2008 as is commonly thought.